1. Field of the Invention
The present invention relates to a magnetic head driving device for an magneto-optical recording apparatus employing a magnetic field modulation system, and the like.
2. Description of the Related Art
In a magneto-optical recording apparatus, a recording medium is irradiated with laser light to raise the temperature of a film formed on the recording medium to provide for perpendicular magnetization. When the temperature of the film rises, the coercive force thereof decreases to allow the film to be magnetized in the direction of an outer magnetic field presented at that time. When the temperature of the film returns to a normal temperature, this magnetization is retained. Information is recorded on the recording medium by changing the laser light or the outer magnetic field as a function of the information to be recorded.
At present, two systems are typically employed for such a magneto-optical recording apparatus. One is an optical modulation system in which laser light is changed in accordance with recording signals. The other is a magnetic field modulation system in which a magnetic field is changed in accordance with recording signals.
In the optical modulation system, a uniform magnetic field is first applied to the recording medium, which is then irradiated with high-energy laser light modulated in accordance with recording signals. As a result, only a spot on the recording medium irradiated with the modulated laser light is magnetized in the direction of the applied magnetic field. On the other hand, in the magnetic field modulation system, the recording medium is first irradiated with uniform high-energy laser light. Then, a magnetic field modulated in accordance with recording signals is applied to a respective spot on the recording medium. As a result, the spot on the recording medium is magnetized in the direction of the applied modulated magnetic field.
In the optical modulation system, when an area on the recording medium where information has already been written is rewritten with different information, it is necessary to initialize the area before rewriting. This ultimately results in a lowering of the access speed of the recording apparatus. On the other hand, in the magnetic field modulation system, since all the areas for rewriting on the recording medium irradiated uniformly with laser light are magnetized in the direction of the magnetic field in accordance with the recording signals, it is possible to directly rewrite new information over the already-written information on the recording medium, without resulting in the lowering of the access speed of the recording apparatus. Accordingly at present, in consideration of the desired increased access speed, the magnetic field modulation system has been more widely employed for the recording apparatus.
In the magnetic field modulation system, it is necessary to reverse or change the magnetic field at high speed in accordance with the recording signals. To achieve this, in a conventional recording apparatus employing this type of system, a magnetic head is positioned closer to the recording medium to minimize the inductance of a head coil in a driver thereof. At the same time, as disclosed in Japanese Laid-Open Patent Publication No. 63-94406, the magnetic head driving device for driving the magnetic head is provided with an auxiliary coil having an inductance sufficiently larger than that of the head coil, thereby assisting the reversal of the magnetic field generated from the head coil.
FIG. 7 shows the above-described conventional magnetic head driving device. The magnetic head driving device includes a first current path 31 and a second current path 32. When a recording signal S is supplied to the magnetic head driving device, main switches 33 and 34 for the first current path 31 and the second current path 32, respectively, are alternately turned ON/OFF in accordance with the recording signal S. When the main switch 33 of the first current path 31 is ON (i.e., closed), a recording current I.sub.X flows into a head coil 39 from a current source 38 of the second current path 32 through a resistor R.sub.12 and an auxiliary coil 36. On the other hand, when the main switch 34 of the second current path 32 is ON, a recording current I.sub.X flows into the head coil 39 in the reverse direction from a current source 37 of the first current path 31 through a resistor R.sub.11 and an auxiliary coil 35. As a result, the head coil 39 generates a magnetic field in accordance with the recording signal S.
In addition, when the main switch 33 of the first current path 31 is ON, a current I.sub.L1 flows from the current source 37 of the first current path 31 through the resistor R.sub.11 into the auxiliary coil 35. Similarly, when the main switch 34 of the second current path 32 is ON, a current I.sub.L2 flows from the current source 38 of the second current path 32 through the resistor R.sub.12 into the auxiliary coil 36. This means that one of the auxiliary coils 35 and 36 is energized with electromagnetic energy by the respective current I.sub.L1 or I.sub.L2 while the other of the auxiliary coils 35 or 36 provides the recording current I.sub.X into the head coil 39. Thus, when the recording signal S is changed to alter the ON/OFF state of the main switches 33 and 34, a current immediately flows into the head coil 39 from either one of the auxiliary coils 35 and 36 energized with electromagnetic energy by the current I.sub.L1 or I.sub.L2 since the head coil 39 has a smaller inductance than the auxiliary coils 35 and 36. As a result, the recording current I.sub.X flowing into the head coil 39 can reverse at high speed.
However, the above-described magnetic head driving device has a disadvantage of power inefficiency, i.e., wasting power. When the main switch 33 of the first current path 31 is ON, the current I.sub.L1 continues to flow only for the purpose of storing electromagnetic energy in the auxiliary coil 35 of the same current path. This also occurs in the second current path 32 for the case when the main switch 34 is ON. To minimize this wasteful power consumption, Japanese Laid-Open Patent Publication No. 63-244402 discloses a magnetic head driving device as shown in FIG. 8. The magnetic head driving device in FIG. 8 includes auxiliary switches 41 and 42 for the auxiliary coils 35 and 36, respectively. The auxiliary switch 41 is turned ON just before the main switch 34 of the second current path 32 is turned ON. The auxiliary switch 41 is then kept ON until the main switch 34 is turned OFF (i.e., opened). On the other hand, the auxiliary switch 42 is turned ON just before the main switch 33 of the first current path 31 is turned ON, and is kept ON until the main switch 33 is turned OFF.
In a magnetic head driving device such as shown in FIG. 8, when the main switch 33 of the first current path 31 is turned ON, at first only the auxiliary switch 42 of the second current path 32 is ON while the auxiliary switch 41 of the first current path 31 is OFF. As a result, the current I.sub.L1 does not flow into the auxiliary coil 35. Then, when the recording signal S is reversed, the auxiliary switch 41 is turned ON, and at the same time, since the main switch 33 is still ON, the current I.sub.L1 starts to flow into the auxiliary coil 35, thus to energize the auxiliary coil 35. After a certain delay time provided by a delay circuit 40, the main switch 33 is turned OFF and the main switch 34 is turned ON. Then, a current from the energized auxiliary coil 35 flows into the head coil 39 to allow the recording current I.sub.X to reverse at high speed.
In this way, the current I.sub.L1 does not continue to flow throughout the period of time when the main switch 33 is ON, thereby not wasting power. Instead, the current I.sub.L1 flows only for a predetermined period of time required for energizing the auxiliary coil 35. This predetermined period is set by adjusting the delay time of the delay circuit 40. Thus, the wasteful use of power can be reduced. The above description is also applicable to the second current path 32 when the main switch 34 is switched from ON to OFF.
However, in the magnetic head driving device of FIG. 7, the auxiliary coils 35 and 36 do not have stored electromagnetic energy at the start of recording. Since the auxiliary coils 35 and 36 have an inductance larger than that of the head coil 39, a longer time is required until the recording current I.sub.X of the head coil 39 reaches a sufficiently high level. More specifically, when the main switch 33 of the first current path 31 is turned ON at the start of recording, several microseconds to several tens of microseconds are required for the recording current I.sub.X supplied into the head coil 39 from the current source 38 to reach a sufficiently high level. This delay is due to a transient response based on the time constant .tau. determined by the resistance of the resistor R.sub.12 and the inductances of the auxiliary coil 36 and the head coil 39. As a result, information recorded at the start of recording tends to be unstable because the magnetic field from the head coil 39 is insufficient.
Furthermore, the magnetic head driving device of FIG. 8 does not completely solve the problem of wasting power. The currents I.sub.L1 and I.sub.L2 flowing into the auxiliary coils 35 and 36, respectively, during the delay time provided by the delay circuit 40 are not effectively used as the recording current I.sub.X flowing into the head coil 39, but instead are wastefully discharged. Moreover, with the increase of the recording rate of the recording medium, the period during which the main switches 33 and 34 are ON will be shorter. To accommodate this shortened ON period, it is necessary to start the supply of the currents I.sub.L1 and I.sub.L2 to energize the auxiliary coils 35 and 36 earlier than when the recording signal S is reversed. As a result, the magnetic head driving device of FIG. 8 will still tend to waste power to substantially the same degree as the magnetic head driving device of FIG. 7.
In view of the abovementioned shortcomings associated with conventional magnetic head driving devices, there is a strong need for a magnetic head driving device in which a head coil thereof can generate a sufficiently high level of magnetic field to effect stable recording even at the start of recording. Moreover, there is a strong need for a magnetic head driving device which does not waste power.